Fluorescent-lamp-dimming circuit

ABSTRACT

A circuit to permit continuous control, over a substantial range, of the power supplied to a negative-resistance load such as a fluorescent lamp. A conventional dimming circuit including a lamp load, a source of cyclically varying voltage and a siliconcontrolled rectifier is modified by the inclusion of an impedance shunted across the silicon-controlled rectifier. The impedance permits improved control of the amount of power being supplied to the lamp load and hence of its intensity of illumination, especially at low levels of power.

United States Patent [72] Inventor John S. Wirtz 3,416,031 12/1968 Franks et a1. 315/199 X Rochester, N.Y. 3,476,976 1 1/1969 Morita et a1. 315/1 9 X 5; QYPA- 3 1969 FOREIGN PATENTS i e une [45] Patented OCL191971 125,240 6/1949 Sweden 315/241 S [73] Assignee Itek Corporation Primary Examiner-Roy Lake Lexington, Mass. Assistant ExaminerPalmer C, Demeo Att0rneys1-lomer 0. Blair, Robert L. Nathans and Gerald H.

Glanzman [54] FLUORESCENT-LAMP-DIMMING CIRCUIT 2 Claims, 6 Drawing Figs. [52] US. Cl 315/199, ABSTRACT; A circuit to permit continuous control, over a 315/DIG- 4 substantial range, of the power supplied to a negative-re- [51] Int. Cl H05b 37/02 sistance load Such as a fluorescent lamp A conventional [50] Fleld of Search :315/199, di circuit including a |amp load, a Source f li ll 100 i 241 S, 100 V, 5 varying voltage and a silicon-controlled rectifier is modified 56 R d by the inclusion of an impedance shunted across the silicon- 1 e erences l 6 controlled rectifier. The impedance permits improved control UNITED STATES PATENTS of the amount of power being supplied to the lamp load and 3,103,618 9/1963 Slater 315/100 D hence ofits intensity ofillumination, especially at low levels of 3,240,989 3/1966 Grunwaldt 315/136 X power.

24 p 1, l 10 l I t, FLUORESCENT 5 l LA M P l l l l SUPPLY l l VOLTAGE 76 l l 480 v RMS GATE PULSE l l SOURCE 20 I l l 78 l .2 l l PAIENTEnn'm 19 I97! /0 FLUORESCENT LAMP SUPPLY VOLTAGE 480 v RMS GATE PULSE SOURCE v :nfliilil P ll JOHN S 140/? T2 INVENTOR FIRING VOLTAGE BY ATTORNEY FLUORESCENT-LAMP-DIMMING CIRCUIT BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to a power control circuit, and more particularly to a continuously variable dimming circuit for negative-resistance lamp loads.

2. Description of the Prior Art In U.S. Pat. No. 3,103,618, granted to S. I. Slater, a continuously variable dimmer for electric lamps is described. Broadly, this dimmer includes the lamp load to be controlled, a source of alternating-current voltage to supply power to said lamp load, and a silicon-controlled rectifier (SCR) to regulate the amount of power being supplied to said lamp load. By controlling the portion of each alternating current cycle during which the SCR conducts, the power being supplied to said lamp load and hence its intensity of illumination may be regulated. When the lamp load consists of one or more ordinary incandescent lamps, such a circuit may be designed to furnish adequate control of light intensity over a substantial range down to a minimum intensity approaching zero illumination.

When the lamp load consists of a fluorescent lamp, or some other negative-resistance load, however, the circuit becomes inadequate. A fluorescent lamp has the property that very little current will flow through it until a certain minimum firing voltage across the lamp is reached. This means that for a fluorescent lamp to be lit at all, when integrated into the circuit of Slater, it would be necessary for the SCR to be rendered conductive during the portion of the applied alternating-current cycle when the voltage is above this minimum firing voltage. If the SCR is caused to conduct at any other time, essentially no current will flow through the lamp because the voltage will be too low, and the lamp will stay dark. From this, it can be seen that by utilizing the circuit of Slater to control a fluorescent lamp, the minimum intensity that can be reached is when the SCR is caused to conduct at the instant in the applied alternating-current cycle just before the voltage decreases below the firing voltage. As will be explained more fully hereinafter, this is a substantial amount of illumination and it would be desirable to permit control of the lamp intensity down to some lower level. Furthermore, at low levels of intensity accurate control of the magnitude of the intensity is very difficult. A very slight change in the time in which the SCR is rendered conductive will cause a substantial change in light intensity. Very small changes in light intensity would be very difficult to manage in this range.

It is, therefore, desirable to provide an improved dimming circuit having particular applicability for use with negative-resistance loads. More particularly, it is an object of this invention to provide a dimming circuit for negative-resistance loads wherein greater control of light intensity, especially at low levels of power, is achieved and wherein a lower minimum intensity may be reached than with conventional circuits.

SUMMARY OF THE INVENTION In accordance with this invention, the above and other desired goals are attained by making use of the property of a fluorescent lamp that once the minimum firing voltage is reached and the lamp lights, it will remain lit until the voltage drops down to zero. Therefore, if it could be ensured that the lamp would automatically light up when the voltage reaches the firing voltage, it would then become possible to cause the SCR to conduct at any time during the alternating-current cycle down to zero voltage, rather than only in the region where the voltage is above the firing voltage, and improved control of the intensity may thus be accomplished. This can be done by connecting an impedance in shunt across the SCR such that there will be a completed circuit regardless of whether the SCR is conducting or not. With the inclusion of this impedance, as soon as the applied alternating-current voltage reaches the point during its cycle that the voltage exceeds the firing voltage of the lamp, current will flow through the lamp and it will light up and remain lit until the voltage drops back to zero at the end of the first l of the cycle. By making this impedance relatively high, the minimum intensity ofthe lamp will be quite low and, in fact, will be lower than the minimum achievable by the conventional circuit. To increase the intensity it would be necessary only to gate the SCR at some instant during the AC cycle to short out the impedance and thus increase the current through the lamp and hence its brightness. Furthermore, since the impedance will permit the SCR to be gated at any instant of the alternating-current voltage cycle down to zero voltage, rather than just in the range above the firing voltage, finer control of the intensity may be achieved.

The above and other features of my invention will be more readily understood with reference to the following detailed description of a preferred embodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a circuit diagram of a dimmer circuit constructed in accordance with this invention.

FIG. 2 is a voltage versus current graph showing the characteristics of a fluorescent lamp, the current being drawn on a logarithmic scale.

FIGS. 3A, 3B, and 4A, 4B, are graphs presented to aid in explaining the operation of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a circuit according to this invention to give continuous control, over a substantial range, of the power supplied to a negative-resistance load.

Connected to the input terminal t of a supply of alternating-current voltage is the load 10, which in this embodiment is considered to be a fluorescent lamp. This lamp is connected to the anode electrode 16 of a silicon-controlled rectifier 14 through a ballast resistor 12. The cathode electrode 18 of the SCR is connected back to the other terminal 1, of the voltage supply. Connected to the gate electrode 20 of the SCR is a gate pulse source 22. The function of pulse source 22 is to apply an appropriate signal to the gate electrode at selected times to initiate conduction by the SCR and it may be of any type well known in the art. A suitable pulse source is illustrated in FIG. 1 of US. Pat. No. 3,358,186. The SCR itself essentially acts as a single-pole, single-throw switch having conducting and nonconducting states. When a signal is applied to its gate electrode, conduction will be initiated by the SCR and it will continue to conduct as long as the voltage from the anode to the cathode remains positive.

This circuit, so far described, as indicated by the broken line box 24 in FIG. I, constitutes a conventional dimming circuit as is known in the art. To understand why such a circuit is inadequate for fine control of a fluorescent lamp at low lamp intensities, it would be helpful to first examine FIG. 2 which shows the voltage versus current characteristics for a fluorescent lamp.

As can be seen in FIG. 2, as an alternating current voltage is first applied across the lamp, a relatively small amount of current will flow through it (line 30). This situation continues until the firing voltage is reached (point 32). At that point, current will begin to surface through the lamp and, in fact, the current would become excessive were it not for the presence of ballast resistor'lZ in series with the lamp to control it. As is indicated by line 34 in FIG. 2, when the firing voltage is reached, an increase in current thereafter results in a decrease in the voltage across the lamp. This is the negative-resistance region of the lamp. In a fluorescent lamp, the current will continue to flow under control of the resistance in series with the lamp until the voltage drops to zero, at which time the voltage must be again raised to the firing voltage to light the lamp. In a normal load, such as an incandescent lamp, an increase in voltage will result in an increase in current throughout its operating range.

Now consider FIG. 3 wherein the utilization of the conventional circuit within box 24 of FIG. 1 as applied to a fluorescence lamp is explained in detail.

FIG. 3A is a voltage versus time curve of the first 180 of the supplied alternating-current cycle. Broken line 40 indicates the firing voltage of the lamp. FIG. 3B shows the corresponding current versus time curve. As indicated previously, the intensity of the lamp will depend on the particular instant along the supplied alternating-current cycle that a signal is applied to the gate electrode of the SCR to cause it to conduct. Thus, the earlier in the cycle that the SCR begins to conduct, the more current will flow through the lamp and the greater will be its intensity. This intensity is indicated by the area under the current versus time curve in FIG. 3B. Now, with respect to a fluorescent lamp, there is the additional factor that for current to flow through the lamp, the SCR must be rendered conductive during the portion of the cycle when the firing voltage is exceeded as indicated by the portion of the curve above line 40 in FIG. 3A. Thus, if the SCR is gated at some point along the cycle before r, or after t the firing voltage will not be present across the lamp and the light will stay off. From this, it is clear that the minimum intensity that can be achieved using the conventional circuit is when the SCR is gated at some point immediately preceding time t,. This intensity is indicated by the area a in FIG. 3B. This area is an appreciable percentage of the total area under the curve.

Now, if it is desired to increase the intensity of the lamp by a slight amount, it would be necessary to gate the SCR a little earlier in the cycle, for example, at time As can be seen in FIG. 38, a slight change in the firing angle will produce a relatively large increase in the lamp intensity, the new intensity being indicated by the area a,+a,. To get only a very small increase in intensity would require time t, to be almost the same as time 1 which would be very difficult to manipulate, especially by manual control means. As indicated above, if it is desired to decrease the lamp intensity a slight amount below that indicated by area a in FIG. 313, it would be necessary to gate the SCR at some time after i However, in such case, the necessary firing voltage will not be present and the light will stay off. Thus it can be seen that when a fluorescent lamp is controlled by the conventional dimming circuit, disadvantages arise as to the minimum intensity available and also as to the capability of achieving very small changes in the intensity.

Returning to FIG. 1, this problem has been solved by adding to the conventional circuit a resistor 26 shunted across the SCR from the anode to the cathode. The effect of this resistor in the circuit can be understood by reference to FIG. 4. FIG. 4A is similar to FIG. 3A and FIG. 4B shows the corresponding current versus time curve with this new circuit.

With a relatively high resistance 26 connected across the SCR, there is a completed circuit through the lamp whether the SCR is conducting or not. Therefore, independently of any action by the SCR, when the applied voltage reaches the firing voltage, there will be a completed circuit and a small amount of current will flow through the lamp. This current will continue to flow until the voltage returns to zero at the end of the one-half cycle, and the resultant very low lamp intensity is indicated by area a in FIG. 4B. As can be seen in the FIG., the current flowing through the lamp is steady with respect to time and is controlled by the sum of the ballast resistance 12 and the shunt resistance 26 in series therewith. By suitable selection of the value of the ballast and shunt resistances, the minimum achievable intensity can be made lower than is possible with the unmodified circuit (area a, in FIG. 3B). If, now, it is desired to make the lamp slightly brighter, it is necessary only to apply a signal to the gate electrode of the SCR. The SCR will be made conductive, shorting out the shunt resistance and increasing the current through and thus the intensity of the lamp. Since the lamp will remain lit, in this case, until the voltage drops to zero, this signal can be applied any time from t, to 1,. To make the lamp slightly brighter, the SCR can be gated, for example, at time t, and the intensity will increase b an amount indicated as a,. From a comparison of FIGS. 3 and 48, it IS clear that when the "me between t, and

l is the same as the time between t, and t. the intensity indicated in FIG. 48 increases by a smaller amount than the intensity indicated in FIG. 38. Moderate changes in the same position of the gating pulse now produce relatively small percentage changes in the composite areas of FIG. 4B. Thus, small adjustments of the lamp intensity are easier to accom plish at these low levels of power.

In carrying out this invention, the following values for the circuit elements in FIG. 1 would be typical: Ballast resistance 12-250 ohms; shunt resistance 24-l ,000 ohms; SCR-300 PIV rating. The shunt resistance should be relatively high compared to the ballast resistance so that the minimum lamp intensity can be kept low.

Although the embodiment above described employed as the load element a fluorescent lamp, it should be understood that this circuit would have equal applicability as a dimming control for any negative-resistance load such as a mercury vapor lamp or an arc lamp. As to the siliconcontrolled rectifier, it is clear that this element is being used basically as a switch and thus any other type of gated switching device such as a triac or a thyratron could readily be substituted therefor.

A further advantage of the present invention is that a less expensive switching device may be utilized in the circuit. In the absence of the shunt impedance, voltages as high as the supply voltage could be impressed across the nonconducting switch resulting in destructive breakdown unless the switch has a sufficiently high off-sustaining voltage. The presence of the shunt impedance makes the voltage across the switch dependent on the current through the shunt impedance and the magnitude of the shunt impedance. Thus, a switch having a lower off-sustaining voltage may be utilized.

I claim:

1. A dimming circuit for negative-resistance lamp loads, said negative resistance lamp loads having the property that substantially no current will flow therethrough until a minimum firing voltage across said load is reached, said circuit comprising:

a. a source of alternating-current voltage;

b. a ballast resistor having a first resistance;

c. switch means having input, output and control terminals;

d. means coupling said source of alternating-current voltage, said negative-resistance lamp load, said ballast resistor, and said switch means in series circuit;

e. a resistor having a second resistance high compared to said first resistance coupled across said switch means between said input tenninal and said output tenninal for causing current to flow through said lamp load during the portions of predetermined altemating-current voltage cycles from the time that the proportional part of the alterhating-current voltage applied across the load reaches the firing voltage until said voltage applied across said load drops to a value which is insufficient to maintain illumination regardless of the conductive state of said switch means; and

f. means coupled to the control terminal of said switch means for applying a signal to initiate conduction by said switch means at any predetermined instant of said portions for controllably increasing the current flowing through said lamp load to regulate the intensity of illumination of said lamp load.

2. A circuit according to claim 1 wherein said negative-resistance lamp load is s fluorescent lamp. 

1. A dimming circuit for negative-resistance lamp loads, said negative resistance lamp loads having the property that substantially no current will flow therethrough until a minimum firing voltage across said load is reached, said circuit comprising: a. a source of alternating-current voltage; b. a ballast resistor having a first resistance; c. switch means having input, output and control terminals; d. means coupling said source of alternating-current voltage, said negative-resistance lamp load, said ballast resistor, and said switch means in series circuit; e. a resistor having a second resistance high compared to said first resistance coupled across said switch means between said input terminal and said output terminal for causing current to flow through said lamp load during the portions of predetermined alternating-current voltage cycles from the time that the proportional part of the alternating-current voltage applied across the load reaches the firing voltage until said voltage applied across said load drops to a value which is insufficient to maintain illumination regardless of the conductive state of said switch means; and f. means coupled to the control terminal of said switch means for applying a signal to initiate conduction by said switch means at any predetermined instant of said portions for controllably increasing the current flowing through said lamp load to regulate the intensity of illumination of said lamp load.
 2. A circuit according to claim 1 wherein said negative-resistance lamp load is s fluorescent lamp. 